The present invention relates to a production method of synthetic silica glass and a thermal treatment apparatus and, more particularly, to a method of producing synthetic silica glass useful as a material for optical components and others used together with ultraviolet lasers, and a thermal treatment apparatus used therein.
Projection exposure systems (steppers) having the structure as illustrated in FIG. 22A and FIG. 22B have been used heretofore in the photolithography technology for printing a microscopic pattern of an integrated circuit on a wafer of silicon or the like.
In the projection exposure apparatus illustrated in FIG. 22A, light from a light source 601 such as a mercury arc lamp or the like is collected by an ellipsoidal mirror 602 and thereafter the light is converted into a collimated beam by a collimator lens 603. Then this collimated beam travels through a fly""s eye lens 604 consisting of an assembly of optical elements 604a of a rectangular cross section as illustrated in FIG. 22B, to form a plurality of light source images on the exit side thereof. An aperture stop 605 having a circular aperture is disposed at this light source image position. Beams from the plurality of light source images are condensed by a condenser lens 606 to uniformly illuminate a reticle R as an object to be illuminated, on a superimposed basis.
A pattern on the reticle R under the uniform illumination by the illumination optical system as described above is projected and printed on a wafer W coated with a resist by a projection optical system 607 consisting of a plurality of lenses. This wafer W is mounted on a wafer stage WS, which is arranged to move two-dimensionally, and the projection exposure apparatus of FIG. 22A carries out the exposure in the so-called step-and-repeat method in which after completion of exposure in one shot area on the wafer, the wafer stage is two-dimensionally moved in order for exposure in a next shot area.
Another method proposed in recent years is a scanning exposure method capable of transferring the pattern of the reticle R onto the wafer W at high throughputs by illuminating the reticle R with a beam of rectangular shape or arcuate shape and scanning the reticle R and the wafer W arranged in conjugate relation with respect to the projection optical system 507, in a certain direction.
In the field of the projection exposure apparatus having such structure, there are desires for higher resolution with recent increase in integration density of LSI (large scale integration). Explaining this with an example of DRAM (dynamic random access memory) out of VLSI (very large scale integration) being a kind of LSI, the capacity thereof increases with development from LSI to VLSI in the following manner; 1Kxe2x86x92256Kxe2x86x921Mxe2x86x924Mxe2x86x9216Mxe2x86x9264Mxe2x86x92256Mxe2x86x921G. With this increase in the capacity, the processing line width of patterns required of the projection exposure apparatus decreases as 10 xcexcmxe2x86x922 xcexcmxe2x86x921xcexcmxe2x86x920.8 xcexcmxe2x86x920.5 xcexcmxe2x86x920.35 xcexcmxe2x86x920.25 xcexcmxe2x86x920.18 xcexcm, respectively.
In order to enhance the resolution of the projection exposure apparatus, the optical members used in their optics need to have high transmittances for the exposure light used. This is because the optics of the projection exposure apparatus are composed of combination of many optical members and even if an optical loss per lens is small accumulation of such losses in the number of optical members used will lead to great decrease in the total transmittance. If optical members with low transmittances are employed they will absorb the exposure light to increase temperatures of the optical members and make refractive indexes inhomogeneous and polished surfaces thereof will undergo deformation because of local thermal expansion of the optical members. This will cause degradation of optical performance.
On the other hand, in the case of projection optical systems, there are demands for high homogeneity of refractive indexes of the optical members in order to obtain finer and clearer projection exposure patterns. This is because the dispersion of refractive indexes causes a lead or lag of light and this largely affects the imaging performance of the projection optical system.
Therefore, silica glass or calcium fluoride crystals with high transmittances for the ultraviolet light and with excellent homogeneity are generally used as materials for the optical members used in the optics of the projection exposure apparatus utilizing the ultraviolet light (of the wavelengths not more than 400 nm). Particularly, in the projection exposure apparatus with the excimer laser used in volume production lines of large capacity VRAM of not less than 16M, 0.25 xcexcm microprocessors, and so on, synthetic silica glass of high purity is used as a material of optical elements for ultraviolet lithography (lens elements used in the illumination optical system or in the projection optical system).
The flame hydrolysis (also called a direct process) is known as a production method of synthetic silica glass. The flame hydrolysis is a method of ejecting a silicon compound as a source material and a combustion gas containing oxygen and hydrogen from a burner to burn the silicon compound in oxyhydrogen flame, thereafter depositing resultant fine particles of silica glass on a target opposed to the burner, and, at the same time as it, vitrifying the silica glass particles to obtain an ingot form of synthetic silica glass.
In general, the flame hydrolysis employs a fabrication system having the structure similar to the so-called Verneuil furnace and the synthesis is carried out with maintaining the in-system temperature at high temperatures of not less than 1000xc2x0 C. The ingot form of silica glass obtained in this method is quickly cooled from the high temperature region of not less than 1000xc2x0 C. to ordinary temperatures by natural cooling, is then subjected to cutting and rounding if necessary, and thereafter is subjected to a thermal treatment step of annealing (slow cooling treatment) or the like, thereby yielding a block material. The block material thus obtained is inspected as to radial index homogeneity, thereafter is processed in lens shape, and is further coated with a coating, so as to be able to be used as an optical member for ultraviolet lithography.
Meanwhile, decrease in wavelengths of light sources used for attainment of higher resolution has been proposed in recent years in the projection exposure apparatus, and, for example, the wavelengths have been decreased to the KrF excimer laser (248 nm) and the ArF excimer laser (193 nm), in place of the g-line (436 nm) and the i-line (365 nm) which have been utilized heretofore.
Since the projection exposure systems using such excimer lasers of short wavelengths are intended for attainment of finer mask patterns, their optics are constructed using materials with higher properties as to the homogeneity of transmittance and refractive index.
There were, however, cases wherein a desired resolution was not attained even if the optics were fabricated by assembling a plurality of materials with high and homogeneous transmittances and refractive indexes.
The present invention has been accomplished in view of the problem of the prior art described above and an object of the invention is to provide a production method of synthetic silica glass useful as a material for the optical members (optical elements) and others forming the optics of the projection optical apparatus (steppers) and capable of attaining sufficiently high imaging performance in the optics and sufficiently high resolution in the projection exposure apparatus even in use with a light source of a short wavelength such as the KrF excimer laser or the ArF excimer laser, and to provide a thermal treatment apparatus used therein.
The inventors have conducted intensive and extensive research in order to accomplish the above object and first found that the imaging performance of the projection optical system and the resolution of the projection exposure apparatus were affected by the optical members and that the imaging performance close to designed performance of the projection optical system and the resolution close to the designed performance of the projection exposure apparatus were able to be attained if the magnitude of double refraction of the optical members, i.e., their birefringence values (absolute values) were not more than 2 nm/cm and if distributions of birefringence values of the optical members were of center symmetry, which is disclosed in Japanese Patent Application Laid-Open No. H08-107060.
With increasing demands for much higher resolution of the projection exposure apparatus, however, there were cases wherein even employment of the above conventional design concept resulted in failure to attain good imaging performance of the projection optical system and good resolution of the projection exposure apparatus when the exposure light was light of shorter wavelengths or when the optical members were of large aperture and large thickness.
Then the inventors have conducted further research and found out that the cause of the failure to attain the projection optical system and the projection exposure apparatus with desired optical performance even with use of the optical members with good transmittance and good index homogeneity was that the optical members had their respective birefringence value distributions different from each other, the different birefringence value distributions were added up in the total optical system when a plurality of optical members were assembled into the projection optical system, and it resulted in disturbing the wavefront of light in the entire optical system and thus greatly affecting the imaging performance of the projection optical system and the resolution of the projection exposure apparatus.
Namely, the evaluation of birefringence values of optical members heretofore was discussed simply by levels of magnitudes (absolute values) thereof and there was no conception to consider the distributions of birefringence values of the optical members. For example, it was common recognition to those skilled in the art that the birefringence values of silica glass members were evaluated by measuring birefringence values at several points near 95% of the diameter of each member and using a maximum thereof as a birefringence value of that member. The inventors, however, discovered that actual distributions of birefringence values were nonuniform, based on detailed measurements to measure distributions of birefringence values of silica glass members.
It was thus verified that even with use of silica glass members with high homogeneity of index the influence of double refraction in the members was not evaluated well by simply managing maximums of birefringence values in the members and that it was very difficult to attain an optical system with desired performance, particularly, in the case of combination of plural members.
Since the double refraction of the entire optical system composed of a plurality of optical members was unable to be evaluated by simply expressing it using only the magnitudes (absolute values) of birefringence values of the individual optical members as described above, the inventors introduced the concept of birefringence values taking account of the direction of the fast axis (i.e., the concept of signed birefringence values) and investigated in detail the influence on optical systems from nonuniform distribution of signed birefringence values of synthetic silica glass. It was found from the investigation that it was difficult to homogenize the distribution of signed birefringence values sufficiently by the conventional annealing known as a means for improving the dispersion of double refraction in production of synthetic silica glass and that if an optical system was constructed of a plurality of optical members of the synthetic silica glass obtained in this way the signed birefringence values would be added up to cause the negative effect on the optical system. Then the inventors discovered that the distribution of signed birefringence values of synthetic silica glass was homogenized well by a specific thermal treatment on the synthetic silica glass obtained by the flame hydrolysis in the production of synthetic silica glass, thus completing the present invention.
Specifically, a production method of synthetic silica glass according to the present invention comprises:
a first step of ejecting a silicon compound and a combustion gas containing oxygen and hydrogen from a burner to hydrolyze the silicon compound in oxyhydrogen flame to generate fine particles of silica glass and thereafter depositing and vitrifying the silica glass particles on a target opposed to the burner to obtain a synthetic silica glass ingot;
a second step of heating the synthetic silica glass ingot obtained in the first step or a synthetic silica glass obtained by cutting of the synthetic silica glass ingot, up to a first retention temperature within a range of not less than 900xc2x0 C., retaining the ingot or the block at the first retention temperature for a predetermined time, and thereafter cooling the ingot or the block at a temperature decrease rate of not more than 10xc2x0 C./h down to a temperature of not more than 500xc2x0 C.; and
a third step of heating the synthetic silica glass ingot or the synthetic silica glass block obtained in the second step up to a second retention temperature within a range of not less than 500xc2x0 C. nor more than 1100xc2x0 C., retaining the ingot or the block at the second retention temperature for a predetermined time, and thereafter cooling the ingot or the block at a temperature decrease rate of not less than 50xc2x0 C./h down to a temperature 100xc2x0 C. lower than the second retention temperature.
The reason why the distribution of signed birefringence values of synthetic silica glass became homogenized by the production method of the present invention was not clear yet, but the inventors deduce the reason as follows.
Namely, the annealing in the conventional production methods of synthetic silica glass is the treatment of heating the synthetic silica glass block mounted on a rotatable table by a heater set on the wall surface and thereafter cooling it at a temperature decrease rate as small as possible, as described in Japanese Patent Application Laid-Open No. H07-113902. It is considered that this method is able to improve the inhomogeneity of distribution of birefringence values caused by temperature distribution in the synthetic silica glass in the cooling process, but it is difficult to improve the inhomogeneity of distribution of birefringence values caused by thermal history during the synthesis, distribution of impurities, and so on, by the above method.
In contrast with it, the inventors infer that the production method of the present invention is capable of improving the inhomogeneity of distribution of birefringence values due to the temperature distribution in the synthetic silica glass in the cooling process and also improving the inhomogeneity of distribution of signed birefringence values due to the thermal history during the synthesis, the distribution of impurities, and so on, by the steps of heating the synthetic silica glass ingot obtained in the first step or the synthetic silica glass block cut out of the synthetic silica glass ingot up to the first retention temperature in the range of not less than 900xc2x0 C., retaining the ingot or the block at the first retention temperature for the predetermined time, then cooling the ingot or the block at the temperature decrease rate of not more than 10xc2x0 C./h down to the temperature of not more than 500xc2x0 C., further heating the ingot or the block up to the second retention temperature in the range of not less than 500xc2x0 C. nor more than 1100xc2x0 C., retaining it at the second retention temperature for the predetermined time, and thereafter cooling it at the temperature decrease rate of not less than 50xc2x0 C./h down to the temperature 100xc2x0 C. lower than the second retention temperature.
In the production method of the present invention, the temperatures in the respective processes of heating, retaining, and cooling mean temperatures on the surface of synthetic silica glass.
In the production method of the present invention, the temperature decrease rates mean average temperature decrease rates between retention temperature and predetermined temperature. Specifically, in the second step according to the present invention, the temperature decrease rate is an average temperature decrease rate in the cooling step from the first retention temperature in the range of not less than 900xc2x0 C. to the predetermined temperature of not more than 500xc2x0 C. on the other hand, in the third step according to the present invention, the temperature decrease rate is an average temperature decrease rate in the cooling step from the second retention temperature in the range of not less than 500xc2x0 C. nor more than 1100xc2x0 C. to the temperature 100xc2x0 C. lower than the second retention temperature, and where the time necessary for the temperature decrease of 100xc2x0 C. after the start of cooling is expressed as t[h], the temperature decrease rate is given by the following equation:
(temperature decrease rate [xc2x0 C./h])=100/t.
In the production method of the present invention, it is preferable that the temperature decrease rate in the third step be not less than 70xc2x0 C./h nor more than 800xc2x0 C./h. When the cooling is implemented at the temperature decrease rate in this range, the distribution of signed birefringence values of synthetic silica glass obtained tends to exhibit better homogeneity.
In the production method of the present invention, it is preferable to use a common furnace in the second step and the third step and carry out the third step continuously without taking the synthetic silica glass ingot or the synthetic silica glass block out of the furnace, after the second step. When the same furnace is used in the second step and the third step and when the third step is carried out continuously without taking the synthetic silica glass ingot or the synthetic silica glass block out of the furnace, after the second step, there are tendencies to be able to efficiently produce the synthetic silica glass with a sufficiently homogenous distribution of signed birefringence values and widen the scope of selection of thermal treatment conditions.
Further, in the production method of the present invention, the third step is preferably a step of successively carrying out the heating, retaining, and cooling with rotating the synthetic silica glass ingot or the synthetic silica glass block. When the heating, retaining, and quick cooling steps are successively carried out with rotating the silica glass in this way, there are tendencies to present better homogeneity of the distribution of signed birefringence values of synthetic silica glass.
A thermal treatment apparatus of the present invention comprises:
a furnace made of a refractor;
a stage capable of carrying synthetic silica glass and moving between a first stage position for letting the synthetic silica glass into the furnace and a second stage position for letting the synthetic silica glass out of the furnace;
a heat generator for heating the synthetic silica glass; and
a driving portion connected to the stage, for moving the stage between the first stage position and the second stage position.
In the thermal treatment apparatus of the present invention, the stage carrying the synthetic silica glass is first moved to the first stage position so as to let the synthetic silica glass into the furnace, and thereafter the synthetic silica glass can be heated and retained at a desired temperature for a desired time by the heat generator. After that, the stage is moved to the second stage position, whereby the synthetic silica glass can be readily taken out of the furnace. Accordingly, when the thermal treatment apparatus of the present invention is applied to the aforementioned production method of the present invention, the thermal treatment including the heating, retaining, and cooling of synthetic silica glass under the desired conditions can be carried out efficiently and surely, so that it becomes feasible to efficiently and surely yield the synthetic silica glass with sufficiently homogeneous distribution of signed birefringence values.
The thermal treatment apparatus of the present invention is preferably constructed to further comprise a rotational driving portion for rotating the stage. When the thermal treatment according to the present invention is carried out with rotating the stage, the distribution of birefringence values of synthetic silica glass obtained tends to exhibit better homogeneity.
Here the notion of signed birefringence value according to the present invention will be described.
The signed birefringence value is a birefringence value with a sign assigned in consideration of the direction of the fast axis defined in the index ellipsoid on the occasion of determining birefringence values of optical members.
More specifically, in a plane normal to the optical axis, centered around an intersection with the optical axis of an optical member, an area subject to circular illumination with a beam is defined as an effective section of an approximate circle, the sign of plus is given to a birefringence value measured when a radial direction from the center being the intersection with the optical axis of the optical member is parallel to the direction of the fast axis in a microregion at a birefringence measuring point on the effective section, and the sign of minus to a birefringence value when perpendicular.
The above way of assignment of the signs to the birefringence values can also be applied to cases wherein a plurality of beams are radiated into the plane normal to the optical axis, centered around the intersection with the optical axis of the optical member. In such cases, the sign of plus is assigned to a birefringence value measured when a radial direction from the center being the intersection with the optical axis of the optical member is parallel to the direction of the fast axis in a microregion around a birefringence measuring point on each of effective sections illuminated with the plurality of beams, and the sign of minus when perpendicular.
Further, the above way of assignment of the signs to the birefringence values can also be applied to cases wherein a beam of the shape other than the circular section, e.g., a beam of a ring section or a elliptic section is radiated into the plane normal to the optical axis, centered around the intersection with the optical axis of the optical member. In such cases, the sign of plus is also assigned to a birefringence value measured when a radial direction from the center being the intersection with the optical axis of the optical member is parallel to the direction of the fast axis in a microregion around a birefringence measuring point on the effective section illuminated with the beam, and the sign of minus when perpendicular.
In the following description, we will describe cases wherein the sign of plus is assigned to a birefringence value measured when a radial direction from the center being the intersection with the optical axis of the optical member is parallel to the direction of the fast axis in a microregion around a birefringence measuring point on an effective section illuminated with light, and the minus sign when perpendicular.
The birefringence values will be described below in further detail with reference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B.
FIG. 1A is a schematic diagram to show directions of the fast axis at birefringence measuring points P11, P12, P13, and P14 located at respective distances r1, r2, r3, and r4 from the center O on the effective section of an optical member L1. For convenience"" sake of description, the birefringence measuring points P11 to P14 are set on a straight line Q1 passing the center O1 and extending along a radial direction in this figure. In the figure, the size of a microregion indicated by a circle at each measuring point is equivalent to an optical path difference at each measuring point. Directions of segments W11, W12, W13, and W14 in these microregions represent directions of the fast axis. Since the directions of the fast axis at the measuring points P11 to P14 all are parallel to the direction of the straight line Q1, i.e., to the radial direction, the birefringence values at the measuring points P11 to P14 are expressed all with the plus sign. The distribution in the radial direction of signed birefringence values A11, A12, A13, and A14 at the measuring points P11 to P14 illustrated in FIG. 1A, obtained as described above, is depicted, for example, as a profile of FIG. 1B.
FIG. 2A is a schematic diagram to show directions of the fast axis at birefringence measuring points P21, P22, P23, and P24 located at respective distances r1, r2, r3, and r4 from the center O2 on the effective section of another optical member L2, similar to FIG. 1A. In this case, since the directions of the fast axis, W21, W22, W23, and W24, all are perpendicular to the direction of the straight line Q2, i.e., to the radial direction, the signed birefringence values A21, A22, A23, and A24 at the measuring points P21 to P24 are expressed all with the minus sign. The distribution in the radial direction of the signed birefringence values A21 to A24 at the measuring points P21 to P24 illustrated in FIG. 2A, obtained in this way, is depicted, for example, as a profile of FIG. 2B.
FIG. 3A is a schematic diagram to show directions of the fast axis at birefringence measuring points P31, P32, P33, P34 and P35 located at respective distances r1, r2, r3, r4, and r5 from the center O on the effective section of another optical member L3, similar to FIG. 1A. In this case, the directions of the fast axis, W31, W32, W33, W34, and W35, at the measuring points P11 to P14 are such that those at the measuring points P31 to P33 are parallel to the direction of the straight line Q3, i.e., to the radial direction, but those at the measuring points P33, P34 are perpendicular to the radial direction. Therefore, the distribution in the radial direction of the signed birefringence values A31 to A35 at the measuring points P31 to P35 is depicted, for example, as a profile of FIG. 3B.